It is well established that cancer patients, as well as animals with experimental cancer, lose body fat during the course of the disease. This is a hallmark of the cachexia produced by this disease. It has been shown also that this loss of body fat, at least in animals bearing experimental tumors, is due primarily to increased fatty acid mobilization. The precise mechanism by which the presence of a tumor results in this increased lipolysis is not understood. The purpose of this study is to determine if there might be an endocrine involvement in this process. The animal model used in the male Sprague-Dawley rat bearing the Walker 256 carcinoma intramuscularly. After some initial difficulties regarding the establishment of a viable tumor line, we have been able to consistently demonstrate increased release of both glycerol and free fatty acids from minced adipose tissue in vitro from tumor-bearing rats relative to controls. Plasma insulin levels in tumor-bearing rats are generally depressed relative to controls. Free adipocytes isolated from tumor-bearing rats convert less (3-3H)-D glucose to lipids under the influence of insulin than do cells from controls. Plasma levels of various lipolytic hormones are also being studied.